An impeller can be used as the rotating component of a pump or generator, and is usually made of a rigid material such as a metal. It transfers energy from a prime mover such as an electric or gasoline powered motor-generator to a fluid being pumped or to increase the rotational rate of the impeller itself. An Axial flow impeller imposes bulk motion, and typical uses include homogenization processes, in which increased fluid volumetric flow rate is important. Impellers are an integral part of an axial-flow pump used in ventricular assist devices to augment or fully replace cardiac function. Radial flow impellers impose a shear stress to the fluid, and are used, for example, to mix immiscible liquids or, in general, when there is a deformable interface to break, or where mixing of viscous fluids is required.
In propeller type impellers the blades are axial thrust-giving elements providing a high degree of swirling in the vessel. The flow pattern generated in the fluid resembles a helix. In paddle type impellers relatively low velocities are attained. Examples are windmills, water wheels, and washing machine agitators. In turbine type impellers the blades are often exposed to high temperature and pressure and also extreme mechanical stress. Such blades are held at both the cord and tip and usually have a relatively low ratio form factor.
The velocity achieved by the impelled fluid transfers into pressure when the outward movement of the fluid is confined by conductor resistance or receiving apparatus such or valves, diverters, and receivers. Linear impellers may have short cylinders with open inlet and outlet through which a fluid moves. Rotating vanes to push the fluid axially provide increased linear flow motion and builds fluid pressure. In a reverse action an impeller may be driven by the flow of a fluid as for instance in a dam turbine so that the energy of the flowing fluid is converted into turbine rotation most often coupled to an electrical generator.